As the information content has increased in recent years, people have come to pay attention to personal communications in which a large volume of analog and digital information is transmitted at a high speed by radio by using high-frequency carriers such as microwaves or millimeter waves. In such communications, there is a demand for development of compact and light microwave and millimeter-wave signal generators having good frequency stability and little phase noise.
A conventional injection locked microwave signal generator is shown in FIG. 14. The injection locked microwave signal generator is made up of a microwave and millimeter-wave amplifier 650 which operates at a fundamental oscillation frequency f', a positive feedback loop 651 consisting of a delay line 652 and a combiner/divider 653, and a microwave and millimeter-wave amplifier 655. In an operation at a free oscillation time, first, a random noise inside the positive feedback loop 651 is amplified by the amplifier 650, so that the noise level of the fundamental oscillation frequency f' becomes high, and the random noise circulates through the positive feedback loop 651. During repetition of the above process, owing to a signal of the fundamental oscillation frequency f' and non-linearity of the amplifier 650, harmonic components n.times.f' (n: integer) of the fundamental oscillation frequency f' grow at a frequency at which the phase rotational angle of the positive feedback loop 651 becomes 360.degree.. Thus, in a steady state, a signal of the fundamental oscillation frequency f' and its harmonics n.times.f' (n: integer) are generated.
Then, by forcibly injecting a signal component having a stable frequency fo (fo=f/m (m: integer)) and a reduced phase noise from the input terminal 660 through the microwave and millimeter wave amplifier 650, the signal having the free fundamental oscillation frequency f' is locked to a signal having a frequency f which is m times the frequency fo of the injection signal. In this manner, it is possible to reduce the phase noise of the free fundamental oscillation frequency f' and stabilize the frequency.
The operation is described below. The forced signal fo injected from the outside causes the signal having the frequency f, which is fo.times.m, to be generated owing to the non-linearity of the microwave and millimeter wave amplifier 655. When the free fundamental oscillation frequency f' is located in the neighborhood of the harmonic fo.times.m of the injection signal (f'.congruent.f), the signal of the free fundamental oscillation frequency f' is pulled in the harmonic fo.times.m (m: integer) of the injection signal into a signal locked to the harmonic signal fo.times.m . Then, the signal is outputted from the output portion 670. Thus, it is possible to reduce the phase noise of the fundamental oscillation frequency f' and stabilize the frequency.
In the method shown in FIG. 14, the phase is controlled by a line length of the positive feedback loop 651 including the delay line 652 and the combiner/divider 653, and the fundamental oscillation frequency f' is determined. When the frequency f' becomes high, the line length of the positive feedback loop 651 becomes short and it becomes difficult to control the fundamental oscillation frequency f'. In such a circuit construction, owing to the characteristic of transmission between D and C of the combiner/divider 653, the injection signal inputted thereto through the amplifier 655 is outputted to the output terminal 670. Thus, the signal taken out from the output terminal 670 includes not only a desired wave but also many undesired waves. Further, basically, the fundamental oscillation frequency f' cannot be varied, although changing the bias point of the amplifier 650 allows the frequency to be slightly changed.
The fundamental oscillation frequency f' can be varied by setting the Q value of the positive feedback loop 651 to a small value thereby to widen the locking range at the injection locking time. But when the Q value of the circuit is small, the fundamental oscillation frequency f' becomes unstable owing to influence by an environmental temperature and other factors. In this case, locking achieved by the signal injection becomes off when the locking range is exceeded. A stable injection locking can be achieved only in the vicinity of the center of the locking range. For such a reason, the circuit construction as mentioned above has problems that it is difficult to obtain a high-frequency wave such as millimeter waves, reduce signals including undesired waves, and vary frequencies.